Nitrobenzene is an important intermediate product of the chemical industry which is required in particular for the preparation of aniline and therefore also for the preparation of di- and polyisocyanates of the diphenylmethane series and the polyurethanes based thereon.
The nitration of benzene with nitric acid to give a crude nitrobenzene has already been the subject of numerous publications and patent applications. The present current processes substantially correspond to the concept of adiabatic nitration of benzene by a mixture of sulfuric and nitric acid (so-called mixed acid). Such a process was claimed for the first time in U.S. Pat. No. 2,256,999 and is described in present-day embodiments, for example, in EP 0 436 443 B1, EP 0 771 783 B1 and U.S. Pat. No. 6,562,247 B2. The processes with an adiabatic reaction procedure are distinguished in particular in that no technical measures are taken to supply heat to or remove heat from the reaction mixture. The starting substances benzene and nitric acid are reacted in a large excess of sulfuric acid, which takes up the heat of reaction liberated and the water formed during the reaction.
The reaction procedure is in general such that the nitric acid and sulfuric acid are combined to form so-called nitrating acid (also called mixed acid). Benzene is metered into this nitrating acid. The reaction products are substantially water and nitrobenzene. Benzene is employed in the nitration reaction at least in the stoichiometric amount, based on the molar amount of nitric acid, but preferably in a 2% to 10% excess. The reaction mixture which is obtained after the reaction zone and is substantially free from nitric acid is fed to a phase separation apparatus in which two phases are formed, the first being called crude nitrobenzene and substantially comprising nitrobenzene, benzene and a quantity of sulfuric acid and water which is dissolved in the nitrobenzene, and the second, called circulating acid, substantially comprising water, sulfuric acid and nitrobenzene dissolved in the sulfuric acid. The circulating acid separated off in the phase separation apparatus is, as described in U.S. Pat. No. 5,313,009, introduced into an apparatus for flash evaporation of the water, in which, by application of a reduced pressure and utilizing the high temperature of the circulating acid which has been achieved by the adiabatic procedure, water is evaporated out of the circulating acid, so that a concentrated sulfuric acid is obtained, the concentration of which substantially corresponds to the concentration before the reaction zone. After the concentration of the sulfuric acid, the sulfuric acid obtained in this way is fed into the reaction without further treatment.
The quality of an adiabatic process for the nitration of aromatic hydrocarbons is defined on the one hand by the content in the product of undesirable by-products of the reaction which are formed by multiple nitration or oxidation of the aromatic hydrocarbon or of the nitroaromatic. The aim in the preparation of nitrobenzene is to minimize the content of dinitrobenzene and of nitrophenols, in particular of trinitrophenol (picric acid), which is to be classified as explosive. On the other hand, the quality of an adiabatic process is defined as it being possible to prepare nitrobenzene without loss of industrial production.
By recycling the sulfuric acid, a sulfuric acid circulation is formed, which comprises the reaction zone, phase separation apparatus, evaporator, buffer tank and connecting lines. It is reported in EP 2 070 907 A1 that metal ions which form sparingly soluble metal sulfates together with sulfate in the sulfuric acid may be present in the sulfuric acid. These metals include the elements Al, Ca, Cr, Mn, Fe, Co, Ni, Cu, Sr, Cd and Ba, in particular Ca and Fe. If the concentration of these metal ions which form sparingly soluble metal sulfates exceeds the solubility limit, metal sulfates precipitate out in the sulfuric acid in the form of a solid and are carried along in the circulation with the sulfuric acid as solids, until they settle and accumulate on a surface or at a narrow point.
It is likewise a known phenomenon that the solubility limit of the metal ions which form sparingly soluble metal sulfates depends greatly on the temperature of the solution, that is to say the temperature of the sulfuric acid. Thus, metal ions dissolve less in cold sulfuric acid than in hot, and consequently metal sulfates are particularly easily produced as a solid in cold sulfuric acid or at points where sulfuric acid is cooled, such as is the case e.g. in heat exchangers. This production of solids in heat exchangers is to be regarded as problematic, since it leads to a covering of the surface of the heat exchanger and therefore to a deterioration in the heat transfer coefficient, and also limits the possible amount flowing through this due to the reduction in the free cross-section of the lines in the heat exchanger.
EP 2 070 907 A1 thus discloses that a cleaning of heat exchangers and lines carrying sulfuric acid to remove solid metal sulfates which have precipitated out is no longer necessary if in the nitration of benzene by a mixed acid containing sulfuric and nitric acid the sulfuric acid obtained again by flash evaporation of water is not recycled completely into the reaction zone as circulating acid, but is partly purged out and replaced by fresh sulfuric acid of low metal ion content.
Periodic flushing of the apparatuses, such as e.g. all those heat exchangers which carry the circulating acid, in order to remove the crystallized metal sulfates from the concentrated sulfuric acid (DE 340 91 17 C2) can thus be dispensed with.
In contrast to the solution disclosed in EP 2 070 907 A1 for avoiding precipitates in heat exchangers and pipelines of a nitration plant, in continuous operation of a nitration plant such as is described in EP 2 070 907 A1 black carbon-containing precipitates which settle in the phase separation apparatus for the crude product, in the crude nitrobenzene condenser and in the neighbouring pipelines have been found.
There was therefore a need for a further improvement in the process for the preparation of nitrobenzene. In particular a smooth operation of the continuous, adiabatic nitration of benzene should be ensured, i.e. in particular the formation of troublesome precipitates, which in the extreme case can lead to the continuous process having to be shut down for cleaning purposes, should be avoided.